1. Field of the Invention
The present invention relates to an organic semiconductor device which comprises an organic semiconductor layer of an organic compound, wherein an organic compound having carrier mobility is used.
2. Description of the Related Art
An inorganic semiconductor, e.g., silicon plays a major role in semiconductor devices, since multiple functions such as high carrier mobility, low dark current, low driving voltage, and a complicated device structure are required for semiconductors used as a diode, a switching device for signal processing, and an amplifying transistor.
In the field of organic semiconductors, an organic electroluminescence device utilizing a photoelectric conversion feature is under development. Furthermore, application of a voltage across the organic semiconductor thin-film increases carrier density, which permits flow of an electrical current between a pair of electrodes provided on an organic semiconductor thin-film. For example, switching over the electrical current that flows in a direction of the organic semiconductor thin-film can be achieved, by means of a source electrode and a drain electrode provided on the organic semiconductor thin-film, and by means of the gate electrode provided therebetween so as to apply a voltage across a thickness direction of the thin-film. Accordingly, research has been conducted on an organic transistor, which permits gradual utilization of the organic semiconductors in technical fields such as transmitting, processing, and recording and displaying of information. In such technical fields, an electrical signal is used for controlling electron carriers and hole carriers within the organic semiconductor at a junction interface between a metal and an organic semiconductor or between one organic semiconductor and another organic semiconductor.
Structure of an organic MOS-TFT using an organic semiconductor thin-film can be provided by, for example, a bottom-contact type or a top-contact type MOS-TFT. Each organic MOS-TFT comprises a gate electrode, a gate insulator layer, a source electrode and a drain electrode, and an organic semiconductor layer, which are formed on a substrate. Material used for the gate electrode is Ni, Cr or the like, and material used for the gate insulator layer is a metal oxide such as SiO2 or SiN, inorganic material such as a nitride, or a resin such as PMMA, and material used for the organic semiconductor layer is pentacene or the like. A layer comprising an elemental metal such as Pd or Au is used for the source electrode and the drain electrode.
However, when the layer comprising an elemental metal such as Pd or Au is used for the source electrode and the drain electrode, the organic semiconductor device has the following problems which impede the practical application of the device:                1) patterning of the electrodes is very difficult, because extremely strong acid such as aqua regia is necessary for wet-etching on Pd or Au, which may damage the base layer such as the gate electrode during the etching process;        2) the device has low reliability because the thin-film of Pd or Au has low adhesion to the base layer film;        3) Pd and Au have a low Clarke number, therefore they are expensive; and        4) Pd has higher electrical resistivity than material normally used for the electrode such as Al.        